A radio transmitter is provided with an amplifier that amplifies power of a transmission signal. In the radio transmitter, in general, in order to increase the power efficiency of the amplifier, the amplifier is operated in the vicinity of the saturation region of the amplifier. However, when the amplifier is operated in the vicinity of the saturation region, nonlinear distortion of the amplifier is increased. Thus, to reduce the nonlinear distortion and meet the standard, such as an adjacent channel leakage ratio (ACLR), the spectrum emission mask (SEM), or the like, the radio transmitter is provided with a distortion compensation device that compensates nonlinear distortion.
A “predistortion (hereinafter, sometimes referred to as “PD”) method” is used as one of distortion compensation method that is used in the distortion compensation device. In the distortion compensation device that uses the PD method, a distortion compensation coefficient that has the inverse properties of the nonlinear distortion of the amplifier is previously multiplied by the transmission signal that is input to the amplifier. Consequently, the nonlinear distortion of the amplifier is canceled out.
Furthermore, in the amplifier with high power efficiency, it is known that a phenomenon called memory effect occurs. The memory effect is a phenomenon in which an output with respect to an input to an amplifier at a certain time is affected. In a distortion compensation method that compensates nonlinear distortion of an amplifier, there is a method that also compensates the memory effect in addition to the nonlinear distortion. With this method, in a transmission signal that has the In-phase component (hereinafter, referred to as “I component”) and the Quadrature component (hereinafter, referred to as “Q component”), a distortion compensation coefficient is determined by using, for example, information on a phase difference between the vector starting from the origin in the IQ coordinate plane to the current transmission signal point and the vector starting from the origin in the IQ coordinate plane to the transmission signal point at a predetermined time before.
Furthermore, as a technology that compensates the memory effect of an amplifier, a distortion compensation method that uses the Volterra series is known. In the distortion compensation method that uses the Volterra series, for example, a transmission signal z(t) that has been subjected to distortion compensation is calculated on the basis of Equation (1) below.
                              z          ⁡                      (            t            )                          =                              ∑                          k              =              1                        K                    ⁢                                    ∑                              l                =                0                            L                        ⁢                                          ∑                                  m                  =                  0                                M                            ⁢                                                a                  klm                                ⁢                                  x                  ⁡                                      (                                          t                      -                      l                                        )                                                  ⁢                                                                                                x                      ⁡                                              (                                                  t                          -                          m                                                )                                                                                                                      k                    -                    1                                                                                                          (        1        )            
In Equation (1) above, K represents the order, L and M represent the depth of a delay, and a represents a distortion compensation coefficient read from each of the order and the delay. The distortion compensation method that uses the Volterra series is the distortion compensation method that uses information on a past transmission signal. Related-art examples are described in International Publication Pamphlet No. WO 2007/046370; International Publication Pamphlet No. WO 2004/045067; Japanese Laid-open Patent Publication No. 2008-294518.
Here, for example, as illustrated in FIG. 16, the case in which the current sample point x(t) of the transmission signal is shifted from a sample point x1(t−Δt) or x2(t−Δt) at a predetermined time before is considered. FIG. 16 is a schematic diagram illustrating the shift of sample points of a transmission signal. In the example illustrated in FIG. 16, the sample points x1(t−Δt) and x2(t−Δt) are located on the same straight line passing through the origin in the IQ coordinate plane. Thus, both the angle formed by a vector 60 of the sample point x(t) and a vector 61 of the sample point x1(t−Δt) and the angle formed by the vector 60 and a vector 62 of the sample point x2(t−Δt) are Δθ.
In a conventional distortion compensation method, a distortion compensation coefficient is determined by using information on a phase difference between the vector of the current sample point of a transmission signal and the vector of the sample point at a predetermined time before. Thus, in the conventional distortion compensation method, for example, as illustrated in FIG. 16, the same distortion compensation coefficient is selected regarding a phase difference even if the current sample point x(t) of the transmission signal is shifted from either one of the sample point x1(t−Δt) at the predetermined time before and x2(t−Δt).
However, in the example illustrated in FIG. 16, if the sample point x(t) is shifted from x1(t−Δt), the amplitude of the transmission signal is increased, whereas, if the sample point x(t) is shifted from x2(t−Δt), the amplitude of the transmission signal is decreased. Thus, a different distortion component is included in the transmission signal between a case in which the sample point x(t) is shifted from x1(t−Δt) and a case in which the sample point x(t) is shifted from x2(t−Δt). However, in the conventional distortion compensation method, a distortion compensation coefficient considering a shift of the amplitude is not specified. Consequently, in the conventional distortion compensation method, it is difficult to improve the distortion compensation performance.
Furthermore, in the distortion compensation method that uses the Volterra series indicated by Equation (1) above, it is possible to perform the distortion compensation considering the shift of past transmission signals; however, an amount of computation is large. Thus, if the distortion compensation method that uses the Volterra series indicated by Equation (1) above is used for a radio transmitter, the size of a circuit becomes large. Thus, it is difficult to apply the distortion compensation method that uses the Volterra series to the radio transmitter reduced its size and power consumption.